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At least 469 records · Page 26
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NASA NTRS1973

A study of the force-field equation for the propagation of galactic cosmic rays

A new development is given of the solution of the equation of the force-field approximation for the propagation of galactic cosmic rays in the interplanetary region. It leads to simpler methods for determining the force-field parameters. A method is given for determining the separable diffusion coefficient from observations of galactic electron spectrum and near-earth electron spectra; it is shown that this diffusion coefficient is not unique but may have a periodic-like dependence upon rigidity; and the method is used to obtain diffusion coefficients for 1965 and 1968. Approximate formulae relating small changes in intensity and diffusion coefficient are developed and some applications of these noted; in one it is shown that the form of, and changes in, diffusion coefficient deduced previously for a neutron monitor event during June-September 1969 are unnecessarily constrained and therefore probably not correct.

Gleeson, L. J.↗
NASA NTRS1974

Isotopic composition of low energy beryllium in the galactic cosmic rays

The multielement charged-particle telescope on the IMP-5 satellite obtained data essentially continuously for more than three years in the interplanetary medium under stable instrument conditions which have made it possible to separate the H, He, and Be isotopes in the galactic cosmic radiation using the technique of double dE/dx vs residual E and particle range measurements. Special emphasis is placed on demonstrating that in the energy range from 50 to 150 MeV/nucleon the instrument clearly resolved Be-7 vs Be-9. A detailed analysis of approximately 100 Be events collected over the mission lifetime yields the ratios (Be-7/Be) = 0.50 plus or minus 0.07, (Be-9/Be) = 0.41 plus or minus 0.10, and (Be-10/Be) = 0.09 plus or minus 0.10. These results are consistent with current models of galactic cosmic ray propagation after taking account of the effects of solar modulation.

Garcia-Munoz, M.↗
NASA NTRS1974

Dust in the galactic center

The strength of the 9.7 microns absorption band in the radiation from the galactic center is used to deduce a range of measures of the visual extinction from 20 to 400 magnitudes. A possible way of arriving at a unique value of the visual extinction consistent with variations in the wavelength dependence of extinction as a function of grain characteristics is indicated. The mass of silicates deduced from the 9.7 microns band appears to imply a gradient of metallicity towards the galactic center.

Greenberg, J. M.↗
NASA NTRS1975

Galactic winds driven by cosmic rays

Cosmic rays traveling through a magnetized plasma with mean velocity greater than the Alfven speed can become coupled to the plasma by the emission of magnetohydrodynamic waves. Using a time-independent, spherically symmetric, hydrodynamic treatment, we show that cosmic rays escaping from a galaxy can carry along thermal gas and produce a 'galactic wind'. Solutions indicate that a typical galaxy can lose mass at a rate of about 1 to 10 million solar masses per year and energy at a rate of about 10 to the 40th to 10 to the 42nd ergs per sec. The possible role of galactic winds in radio and X-ray sources is explored. A model is proposed to explain the X-ray emission observed from clusters of galaxies.

Ipavich, F. M.↗
NASA NTRS1975

Energy dependence of the Si/Fe ratio in the galactic cosmic rays

New measurements are reported of the Si/Fe ratio and the differential energy spectra of galactic cosmic-ray Si and Fe, taking into account the energy range from 35 MeV to 4 GeV per nucleon. The measured data were obtained with the aid of satellite and balloon-borne instrumentation. It is found that for the region below 1 GeV per nucleon the local interstellar spectra computed are consistent with previous galactic cosmic-ray propagation models obtained by Shapiro and Silberberg (1970) and Mason (1972).

Garcia-Munoz, M.↗
NASA NTRS1975

Isolated Stroemgren spheres as a source of galactic H alpha emission

The isolated H alpha emission features reported by Reynolds et al. are identified with Stroemgren spheres surrounding observed O stars; the root mean square electron densities and Stroemgren radii are calculated from the observed emission measure and stellar type. The density has an average value of 2 per cu cm but increases to 3.3 per cu cm for stars within 200 pc of the galactic plane. An analysis of the Catalogue of Galactic O Stars shows that enhanced H alpha emission is observed around all known O stars. The diffuse component of H alpha emission is what one would expect from the line-of-sight overlapping of isolated Stroemgren spheres surrounding B stars and appears to correlate with features on extinction maps, as derived. There is no compelling reason to suggest any large-scale intercloud ionization from H alpha data alone.

Elmergreen, B. G.↗
NASA NTRS1975

Optical studies of 10 high galactic latitude X-ray sources

Optical studies are reported for a representative sample of 10 unidentified high galactic latitude X-ray sources that are north of declination minus 25 deg and that have 3U error boxes less than or of the order of 0.5 square degrees. The source 3U 1410-03 is identified with a peculiar emission-line spiral galaxy, NGC 5506, and a distance group 4 rich cluster of galaxies, Abell 2142, is suggested as a possible identification for 3U 1555+27. It is shown that very few of the unidentified high galactic latitude sources are quasi-stellar sources brighter than 17.5 mag, although one possible X-ray quasar, NAB 0137-01, is reported.

Bahcall, J. N.↗
NASA NTRS1976

Diffuse galactic gamma ray lines

The origin and observability were studied of diffuse gamma ray line emission from the galaxy. It was found that such lines could be formed by nuclear excitation interactions of low energy cosmic rays with both interstellar gas and dust grains. The gamma ray emission lines from deexcitation of grain nuclei are sharp with Doppler widths of the order of 10 kev or less; the lines from gas nuclei are also relatively sharp with widths of the order of 100 kev for the most intense line; and the lines from cosmic ray nuclei are broad with widths of the order of several hundred kev. A detailed evaluation is presented of the production rate of the 4.44 Mev line for a variety of assumed cosmic ray spectra. Results are compared with reported galactic gamma ray line intensities and it is concluded that the measurements are consistent with a low energy cosmic ray density which increases toward the galactic center in proportion to the molecular gas density.

Lingenfelter, R. E.↗
NASA NTRS1976

The structure and content of the galaxy and galactic gamma rays. A review

The International Gamma Ray Symposium entitled 'The Structure and Content of the Galaxy and Galactic Gamma Rays', held at the NASA Goddard Space Flight Center in June, 1976, is briefly reviewed. Highlights from presentations on galactic structure, discrete sources, and low energy gamma rays are described. New data and their interpretations are also summarized.

Crannell, C. J.↗
NASA NTRS1975

Galactic gamma radiation from cosmic rays concentrated in spiral arms

Extending the model proposed by Bignami et al. (1975), and by Fichtel et al. (1975a) for the production of the galactic high energy gamma ray distribution observed by SAS-2 to lower energies indicates the radiation is dominated by the bremsstrahlung emission of cosmic ray electrons traversing the interstellar gas. Although secondary electrons contribute only about 15% to the 10-30 MeV gamma ray emission in the solar vicinity, their contribution in the model is proportional to the third power of N, where N(r, gal. long., gal. lat.) is the total interstellar gas density, as compared to the square of N for the case of the primary components, and hence their relative importance increases in high density regions. Gamma-ray observations at these energies when compared to those at high energies (above 100 MeV) may provide a means for mapping the ratio of cosmic ray electrons to nucleons throughout the galaxy without the necessity of invoking models for the galactic magnetic field.

Kniffen, D. A.↗
NASA NTRS1975

Distribution of cosmic gamma rays in the galactic anticenter region as observed by SAS-2

The high energy (above 35 MeV) gamma ray telescope flown on the second Small Astronomy Satellite has collected over one thousand gamma rays from the direction of the galactic anticenter. In addition to the diffuse galactic emission the distribution indicates a strong pulsed contribution from the Crab nebula with the same period and phase as the NP0532 pulsar. There also seems to be an excess in the direction of (gal. long. ? 195 deg; gal. lat ? +5 deg) where there is a region containing old supernova remnants. Search for gamma ray pulsations from other pulsars in the region do not show any statistically significant signal. The general intensity distribution of the gamma rays away from the plane appear to be similar to nonthermal radio emission brightness contours.

Kniffen, D. A.↗
NASA NTRS1975

The relative abundances of galactic cosmic ray Be-7, Be-9, and Be-10

The isotopic composition of galactic cosmic ray Be near 100 MeV/nucleon was measured in interplanetary space with high-resolution solid-state telescopes on the IMP-7 and IMP-8 satellites during 1973-1974. These two independent measurements yield Be-10/Be ratios above 10%. Taken together with the galactic propagation model described in the text, these results show that the cosmic ray 'clock' lifetime (most probable value of roughly 20 million years) is significantly greater than that deduced for a nominal interstellar gas density of about 1 to 3 atoms/cu cm.

Garcia-Munoz, M.↗
NASA NTRS1976

New galactic population class

It has been found that galactic gamma rays come from regions in which molecular hydrogen is dense and in which 'population I' objects are predominantly found. An interpretation of the existence of such regions leads to a proposal for a change in the Baade classification scheme. 'Population I' is to include all the objects associated with the H2 clouds. A new class, 'population 0' is to refer to the regions, relatively richer in atomic hydrogen, which lie further from the galactic center.

Stecker, F. W.↗
NASA NTRS1976

Solar modulation and a galactic origin for the anomalous component observed in low-energy cosmic rays

The current theory of solar modulation can be used to argue that the cosmic-ray component at low energies, which is observed to have an anomalous composition, is not of galactic origin, i.e., it is not a component of the galactic cosmic-ray flux. The current theory predicts, from quite general considerations, that an unreasonably large intensity of cosmic rays, by many orders of magnitude, would be required in the interstellar medium to account for the observed fluxes. Conceivably, the current modulation theory could be modified so that only reasonable interstellar fluxes are predicted. One such modification involves an unusual scheme for particle diffusion in the interplanetary medium. Particles are assumed to diffuse not as a result of their own motion among small-scale irregularities in the magnetic field, but rather in a process in which they are trapped between time-varying constrictions in the large-scale field.

Fisk, L. A.↗
NASA NTRS1976

Consequences of a lifetime greater than 10 to the 7th power years for galactic cosmic rays

The implications of recent determinations of the cosmic-ray lifetime are discussed. It is concluded that the observations are consistent with a 'dynamical halo' model in which cosmic rays are confined in an outward-moving galactic halo by self-generated hydromagnetic waves. Alternative models which do not incorporate a halo, but which have the cosmic rays propagate in regions of reduced density in the galactic disk, are also briefly discussed.

Jokipii, J. R.↗
NASA NTRS1977

Diffuse galactic gamma ray lines

The production rate of 4.44-MeV line for a variety of assumed cosmic ray spectra is evaluated. These results are compared with reported galactic gamma-ray line intensities and are consistent with a low energy cosmic ray density which increases toward the galactic center in proportion to the molecular gas density.

Lingenfelter, R. E.↗
NASA NTRS1977

Gamma rays and large scale galactic structure

Gamma ray astronomy is now beginning to provide a new look at the galactic structure and the distribution of cosmic rays, both electrons and nucleons, within the galaxy. The observations are consistent with a galactic spiral-arm model in which the cosmic rays are linearly coupled to the interstellar gas on the scale of the spiral arms. The agreement between the predictions of the model and the observations for regions of the plane where both 21-cm and 2.6-mm CO surveys exist emphasizes the need to extend these observations to include the entire plane. Future gamma-ray observations with more sensitivity and better angular resolutions, combined with these radio surveys, should shed new light on the distribution of cosmic rays, the nature of the galaxy, and the location and intensity of the spiral arms.

Kniffen, D. A.↗